The use of solid state imaging devices such as CMOS and other image sensor arrays are found in a growing number of electronic products including digital cameras and some cell phones. In digital cameras in particular, there has been a push to capture images with seemingly greater and greater resolution which has necessitated the development of image sensor chips having increased numbers of pixels.
Each pixel in an image sensor array converts the light incident on that pixel into electronic charge which is accumulated for a fixed period of time. The accumulated charge is then transferred out of the array in a preselected order and typically detected as a voltage signal.
The leakage current in the pixel is often referred to as the dark current of the device, i.e., the current through the light sensitive element in the absence of any light. The lowest light intensity that is detectable by this photosensitive device is dictated by the noise in the device and the dark current. Thus, it is important to keep such leakage currents as small as possible.
For economic reasons in the fabrication of integrated circuits, it is important to keep the size of the integrated circuit die small. As such, increasing the number of pixels in an image sensor array results in an effort to reduce the size of individual pixels, so as to keep the overall die size small. The smaller the size of a given pixel, the less charge that will be photogenerated in a fixed time period. The smaller signal thus created is not accompanied by a proportional decrease in the dark current with a resultant decrease in the signal-to-noise ratio.
In addition, signals from different pixels of an image sensor may pass through different processing channels. For example, a typical color image sensor will have three channels corresponding to the red, green, and blue colors. These processing channels will have differing design gains due to differing sensitivities of the various pixels to differing color bands. In addition to the design difference, however, the different processing channels will also have variations due to processing variations, operating temperatures, and the like.